The present invention relates to a mass spectrometer and a method of mass spectrometry. The mass spectrometer is preferably arranged for charge reduction or charge stripping of Electron Transfer Dissociation (“ETD”) product or fragment ions via Proton Transfer Reactions (“PTR”) with gaseous neutral superbase reagents.
Electrospray ionisation ion sources are well known and may be used to convert neutral peptides eluting from an HPLC column into gas-phase analyte ions. In an aqueous acidic solution, tryptic peptides will be ionised on both the amino terminus and the side chain of the C-terminal amino acid. As the peptide ions proceed to enter a mass spectrometer the positively charged amino groups hydrogen bond and transfer protons to the amide groups along the backbone of the peptide.
It is known to fragment peptide ions by increasing the internal energy of the peptide ions through collisions with a collision gas. The internal energy of the peptide ions is increased until the internal energy exceeds the activation energy necessary to cleave the amide linkages along the backbone of the molecule. This process of fragmenting ions by collisions with a neutral collision gas is commonly referred to as Collision Induced Dissociation (“CID”). The fragment ions which result from Collision Induced Dissociation are commonly referred to as b-type and y-type fragment or product ions, wherein b-type fragment ions contain the amino terminus plus one or more amino acid residues and y-type fragment ions contain the carboxyl terminus plus one or more amino acid residues.
Other methods of fragmenting peptides are known. An alternative method of fragmenting peptide ions is to interact the peptide ions with thermal electrons by a process known as Electron Capture Dissociation (“ECD”). Electron Capture Dissociation cleaves the peptide in a substantially different manner to the fragmentation process which is observed with Collision Induced Dissociation. In particular, Electron Capture Dissociation cleaves the backbone N—Cα bond or the amine bond and the resulting fragment ions which are produced are commonly referred to as c-type and z-type fragment or product ions. Electron Capture Dissociation is believed to be non-ergodic i.e. cleavage occurs before the transferred energy is distributed over the entire molecule. Electron Capture Dissociation also occurs with a lesser dependence on the nature of the neighbouring amino acid and only the N-side of proline is 100% resistive to Electron Capture Dissociation cleavage.
One advantage of fragmenting peptide ions by Electron Capture Dissociation rather than by Collision Induced Dissociation is that Collision Induced Dissociation suffers from a propensity to cleave Post Translational Modifications (“PTMs”) making it difficult to identify the site of modification. By contrast, fragmenting peptide ions by Electron Capture Dissociation tends to preserve Post Translational Modifications arising from, for example, phosphorylation and glycosylation.
However, the technique of Electron Capture Dissociation suffers from the significant problem that it is necessary simultaneously to confine both positive ions and electrons at near thermal kinetic energies. Electron Capture Dissociation has been demonstrated using Fourier Transform Ion Cyclotron Resonance (“FT-ICR”) mass analysers which use a superconducting magnet to generate large magnetic fields. However, such mass spectrometers are very large and are prohibitively expensive for the majority of mass spectrometry users.
As an alternative to Electron Capture Dissociation it has been demonstrated that it is possible to fragment peptide ions by reacting negatively charged reagent ions with multiply charged analyte cations in a linear ion trap. The process of reacting positively charged analyte ions with negatively charged reagent ions has been referred to as Electron Transfer Dissociation (“ETD”). Electron Transfer Dissociation is a mechanism wherein electrons are transferred from negatively charged reagent ions to positively charged analyte ions. After electron transfer, the charge-reduced peptide or analyte ion dissociates through the same mechanisms which are believed to be responsible for fragmentation by Electron Capture Dissociation i.e. it is believed that Electron Transfer Dissociation cleaves the amine bond in a similar manner to Electron Capture Dissociation. As a result, the product or fragment ions which are produced by Electron Transfer Dissociation of peptide analyte ions comprise mostly c-type and z-type fragment or product ions.
One particular advantage of Electron Transfer Dissociation is that such a process is particularly suited for the identification of post-translational modifications (“PTMs”) since weakly bonded PTMs like phosphorylation or glycosylation will survive the electron induced fragmentation of the backbone of the amino acid chain.
It is known to perform Electron Transfer Dissociation by mutually confining cations and anions in a 2D linear ion trap which is arranged to promote ion-ion reactions between reagent anions and analyte cations. The cations and anions are simultaneously trapped within the 2D linear ion trap by applying an auxiliary axially confining RF pseudo-potential barrier at both ends of the 2D linear quadrupole ion trap.
Another method of performing Electron Transfer Dissociation is known wherein a fixed DC axial potential is applied at both ends of a 2D linear quadrupole ion trap in order to confine ions having a certain polarity (e.g. reagent anions) within the ion trap. Ions having an opposite polarity (e.g. analyte cations) to those confined within the ion trap are then directed into the ion trap. The analyte cations will react with the reagent anions already confined within the ion trap.
It is known that when multiply charged (analyte) cations are mixed with (reagent) anions then loosely bound electrons may be transferred from the (reagent) anions to the multiply charged (analyte) cations. Energy is released into the multiply charged cations and the multiply charged cations may be caused to dissociate. However, some of the (analyte) cations may not dissociate but may instead be reduced in charge state. The cations may be reduced in charge by one of two processes. Firstly, the cations may be reduced in charge by Electron Transfer (“ET”) of electrons from the anions to the cations. Secondly, the cations may be reduced in charge by Proton Transfer (“PT”) of protons from the cations to the anions. Irrespective of the process, an abundance of charged reduced product ions are observed within mass spectra and give an indication of the degree of ion-ion reactions (either ET or PT) that are occurring.
In bottom-up or top-down proteomics Electron Transfer Dissociation experiments may be performed in order to maximize the information available by maximizing the abundance of dissociated product ions within mass spectra. The degree of Electron Transfer Dissociation fragmentation depends upon the conformation of the cations (and anions) together with many other instrumental factors. It can be difficult to know a priori the optimal parameters for every anion-cation combination from an LC run.
One problem with known Electron Transfer Dissociation arrangements is that the fragment or product ions resulting from the Electron Transfer Dissociation process tend to be multiply charged and tend also to have relatively high charge states. This is problematic since highly charged fragment or product ions can be hard for a mass spectrometer to resolve. The parent or analyte ions which are fragmented by Electron Transfer Dissociation may, for example, have a charge state of 5+, 6+, 7+, 8+, 9+, 10+ or higher and the resulting fragment or product ions may, for example, have a charge state of 4+, 5+, 6+, 7+, 8+, 9+ or higher.
It is desired to address the problem of ETD product or fragment ions having relatively high charge states which is problematic for a mass spectrometer to resolve.